1. Field of the Invention
The present invention relates to navigation and position determining systems, and more particularly to navigation utilizing data from the global positioning system and an attitude and heading reference system.
2. Description of the Related Art
Aircraft have traditionally used an inertial reference system (IRS) with motion sensors connected to a processor that continuously tracked the position and velocity (direction and speed) of the aircraft without using external references. The inertial reference system was initialized on the ground by the flight crew entering the known position coordinates of the aircraft, such as the longitude, latitude, and altitude of the airport at which the aircraft is parked. As the aircraft moved thereafter, the inertial reference system updated the position and velocity by integrating information received from the motion sensors on the aircraft. The motion sensors usually included three accelerometers that measured linear acceleration along three orthogonal axes, and a trio of gyroscopes that measured angular velocity along the three axes. The aircraft velocity and change in position was derived from the accelerometer and gyroscope signals. The change in position along with a previously determined position are employed to derive a new position for the aircraft.
Even though the typical inertial reference system employed relatively expensive and sophisticated accelerometers and gyroscopes, it suffered from integration drift. Integration drift is small errors in the measurement of acceleration and angular velocity that become integrated into progressively larger aircraft velocity and position errors. Thus, over the course of a trip, the inertial reference system's indication of position will deviate from that actual position of the aircraft.
Because of that drift, some present day inertial reference systems are corrected by data from the global positioning system (GPS). The GPS uses a constellation of earth orbiting satellites that continuously transmit messages via microwave signals containing the time at which the message was sent and ephemeris data regarding the precise orbit for the satellite. A GPS receiver onboard the aircraft uses the arrival time of each message, the time it was sent, and the propagation rate of the microwave signal, to calculate the separation distance between the aircraft and the satellite. The position of the satellite is determined from its ephemeris data. This information tells the GPS receiver that the aircraft is located on an imaginary spherical surface centered at the satellite and having a radius equal to the separation distance. Using similar data from a second satellite, the GPS receiver determines that the aircraft also is located on a second imaginary spherical surface and more specifically on the circular intersection of the two spherical surfaces. The spherical surface related to a third satellite intersects the circle at two points, only one of which often is a possible position for the aircraft as the other point may be too far from the earth. Nevertheless, the data from a fourth satellite eliminates one of those two points, confirming the precise position of the aircraft. The three dimension GPS position then was used to correct the drift error of the inertial reference system.
However, if GPS signals from a sufficient number of satellites were not received, the inertial reference system drift error no longed could be dynamically corrected. For example, atmospherical and astronomical conditions adversely affect the accuracy of the GPS and other factors adversely affect GPS signal reception. If the GPS information is lost, the accuracy of the inertial reference system degraded over time.
Aircraft also have employed an attitude and heading reference system (AHRS) that as the name implies provided information regarding the aircraft's attitude (i.e., pitch and roll) and the aircraft's heading. This system also sensed acceleration along three orthogonal axes, and angular velocity along the three axes, however it did not determine the position and velocity of the aircraft.
Whereas inertial reference systems utilized relatively expensive and thus accurate accelerometers and gyroscopes, the attitude and heading reference systems employed very inexpensive as less accurate versions of those types of devices. For example, an AHRS may use small micro electromechanical systems (MEMS) type accelerometers, consisting of little more than a cantilever beam with a proof mass. Available units comprise MEMS accelerometers mounted orthogonally to each other in a common package for three axis acceleration sensing.